The present invention relates to remediation of volatile contaminants from soil, particular to soil remediation using joule heating technology, and more particularly to electro-osmotic infusion of ground water or chemically tailored electrolyte to enhance, maintain or recondition electrical conductivity for the joule heating remediation technique.
Removal of volatile contaminants, such as gasoline products and industrial wastes, from the soil is being carried out using the known joule heating remediation technique, wherein heating electrodes are utilized to vaporize and drive the contaminants toward an extraction well. A major problem with the joule heating technique is the non-uniform heating of near-electrode soils because of electrode geometry and electrical current distribution. High electrical current densities occur in the soil around the electrodes, which have diameters of less than one foot, when potential differences of several hundred volts are applied to an electrode array. This can result in extremely nonuniform heating of the target soil layer or formation and in total dryout of the near-electrode soil and the complete loss of electrical continuity between electrodes long before significant heating of the far-electrode soil region has been achieved.
The typically low permeability of the joule heating target layer, such as clay, makes it very difficult, if not impossible, to mitigate such effects by simply pumping water into the electrode wells. It has been found that temperatures of many hundreds of degrees can occur near a heating electrode while sub-boiling temperatures exist only a few feet away. Even though electrodes may be separated by fifty or more feet across a zone of contamination, the actual heating effects may be quite localized, involving the soil only a few feet from the electrode. It has also been found that simply adding water to an electrode well to prevent dryout may compound rather than mitigate the problem. First, the water added cannot flow into the target soil layer to prevent dryout because of the low permeability of that layer. Second, adding water to a hot well produces steam at the well that represents thermal energy lost to the target soil layer. Third, the steam produced can flow into adjacent higher permeability layers so that the temperature and saturation of these layers increases, which increases the electrical conductivities of these adjacent layers, causing a shunt of current around the target layer and subsequent decrease in the heating of that layer.
The above problems relating to soil remediation by joule heating techniques are overcome by the present invention which involves a process in which electro-osmotic infusion is utilized to provide a supply of electrolyte or ground water to the zone of intense heating around the joule heating electrodes. This is achieved by applying a direct current (dc) electrical field between electrode wells emplaced in the target soil layer or formation. The application of the dc electric field interacts with the pore scale distribution of ions to produce a net flow of pore fluid toward electrodes having one of the polarities. Induced flows by electro-osmosis are known and have been used extensively in civil engineering applications to dewater and stabilize soils. The electro-osmotic induced flows can be used to infuse electrolyte with enhanced ionic conductivity into the vicinity of the electrodes, maintain the local saturation of the near-electrode soil region, and resaturate a partially dried out zone.